Transceiver

A processor talks in logic levels, a pin at 0V or at its rail. A bus standard rarely does. CAN uses a differential voltage across two wires, RS-485 uses another, USB another again, and none of them are something a general-purpose logic pin can drive or safely receive.

The transceiver sits between the two and translates.

flowchart TB
    M["processor<br/>TXD / RXD, logic levels"] <--> X["transceiver"]
    X <--> B["the bus<br/>CANH / CANL, differential"]

It is the reason a bus interface is three or four nets at the processor and two on the connector, and the reason a board can carry a bus without the processor knowing anything about the bus’s electrical spec.

Because it faces the outside world, a transceiver is usually the part that also carries the bus’s protection, or sits directly behind it.

Where the course teaches it: chapter 10, what an interface requires beyond two wires.